A method to realize the fine frequency-tuning steps using tiny capacitors instead of Metal-Insulator-Metal (MIM) capacitors is proposed for a digitally controlled oscillator (DCO). The tiny capacitors are realized by the coplanar transmission lines which are arranged unsymmetrical in a 6 metal layers (M6) foundry of 0.18µm CMOS technology. These transmission line based capacitors are designed by using electro-magnetic field simulator, and co-designed by using SPICE simulator. Finally, these capacitors are employed to design 15bit DCO and fabricated the proposed DCO in 0.18µm CMOS technology, and tested. The measured phase noise of DCO was -118.3dBc/Hz (@1MHz offset frequency), and the oscillating frequency tuned from 4.86GHz to 5.36GHz in the minimum frequency-tuning step of 18kHz.